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Re: Aquatic Plants Digest V3 #40

> Date: Wed, 21 Jan 1998 15:46:59 -0700 (MST)
> From: George Booth <booth at hpmtlgb1_lvld.hp.com>
> Subject: pH Probe End-of-Service

Just an addendum to George's post on pH probe death and how to detect it.  If
your pH monitor happens to have a "mV" or millivolts setting, the quality of
your probe is easy to check.  A pH probe is basically a sensitive voltometer
and converts the reading in mV that it gets to pH, depending on the calibration
that the user sets based on temperature and standardization to solutions of
known pH.  A solution of neutral pH should have a mV reading of 0.0.  So, if
your probe monitor has an mV display option, check it out in a neutral
buffered standard solution.  In general, pH probes in a neutral solution should
read between -30 mV and +30 mV if they are clean and accurate.  If the probe
does not read in this range, there are ways to clean it most of the time.
Many probes have refill ports for the internal reference solution (usually KCl
saturated with AgCl).  First, try emptying this solution from your probe and
refilling.  Repeat a few times if there are a lot of crystals in the body of
the probe (these are KCl and are harmless, but are an indication that your
internal solution is old and needs changing).  If this does not do the trick,
you probably have a clogged up reference port (the small pore-like thing on
the side of the probe).  There are a number of ways to clean this.  I prefer a
30 min. soak in 0.1M EDTA to remove inorganics.  Ammonia (white) is supposed to
work, but I have not tried it. 0.1N HCl also works
well for both chemical deposits and bacterial goo, and if you have some access
to pepsin, 1/4 tsp (sorry for the non-metric)/100 ml 0.1N HCl will get rid of
protein deposits.  If you have access
to a laboratory, a hot solution of Ammonium hydroxide is the real industrial
strength way to go about cleaning the probe (email me if you want the
specifics).  DONT use this on plastic bodied probes, though.  Other things to
try, but NOT on plastic probes, are acetone and methanol (both should be pure,
no fingernail polish remover or shoe polish <G>).  These do a good job on lipid
based deposits. If you do
use any of these methods to clean your reference port, make sure to empty the
internal reference solution and refill afterwards.  You should also
pressure fill the body of the probe by firmly placing a squeeze bottle of
KCl/AgCl internal solution firmly against the fill port and squeezing until
a bead of the stuff appears on the reference port.  Now, I mostly
have experience with laboratory grade probes and am assuming that all probes,
including PetWarehouse specials, have all of the components that I described,
including a fill port and a reference port.  If this is not the case, sorry,
but maybe we can figure something else out.  Bottom line is, if you only need
accuracy to within .1 or so pH points (I haven't found the need for more), you
can keep cleaning and reusing probes for a fairly long time.  Sorry for the
long post, and please contact me if I can clear up any vagueness.

Oh, and I just remembered.  Sometimes a one hour stir in warm (60C) pH 4.0
standard solution works wonders for mildly crudded up probes.


                                        Dave Gauthier
                                        gauthie9 at pilot_msu.edu